Method and apparatus for communicating information between lock stepped processors

ABSTRACT

An apparatus for communicating between lock step is incorporated on two or more processors operating in a lock step mode. Each of the processors includes processor logic to execute a code sequence, and an identical code sequence is executed by the processor logic. The apparatus further includes a processor-specific resource referenced by the code sequence. A multiplexer is coupled to the processor-specific resource, and is controlled to read data based on the identification. Coupled to the processors is a lock step logic block operable to read and compare the output of each of the processors. The lock step logic determines if operation of the processors is in a lock step mode or in an independent processor mode. Such determination may be made by the lock step logic turning off, for example.

TECHNICAL FIELD

The technical field is mechanisms and methods for communications betweenprocessors in a multi-processor computer system.

BACKGROUND

Advanced computer architectures may employ multiple processors. Someadvanced computer architectures may employ multiple microprocessors onone silicon chip. In a typical application, two microprocessors may beimplemented on a single silicon chip, and the implementation may bereferred to as a dual core processor. Two or more of the multiplemicroprocessors may operate in a lock step mode, meaning that each ofthe lock stepped microprocessors process the same code sequences, andshould, therefore, produce identical outputs.

FIG. 1 is a diagram of a prior art dual core processor that uses lockstep techniques to improve overall reliability. In FIG. 1, a computersystem 18 includes a dual core processor 20 having a single silicon chip21, on which are implemented microprocessor core 22 and microprocessorcore 24. To employ lock step, each of the microprocessor cores 22 and 24process the same code streams. The cores 22 and 24 are coupled to a lockstep logic 26. An event that causes a loss of lock step can occur oneither or both of the microprocessor cores 22 and 24. An example of suchan event is a data cache error. A loss of lock step, if not promptlycorrected, may cause the computer system 18 to “crash” or be “disabled.”In addition, because the microprocessor cores 22 and 24 are running thesame code sequence, by definition, communications between themicroprocessor cores 22 and 24 may not be possible without causing aloss of lock step. For example, the usual means for communicationbetween processors involves load and store sequences, which must bedifferent between two processors, and which consequently will cause aloss of lock step.

SUMMARY

An apparatus, and a corresponding method, for communicating between lockstep processors is disclosed. The apparatus is incorporated on two ormore processors operating in a lock step mode. Each of the processorscomprise processor logic to execute a code sequence, wherein anidentical code sequence is executed by the processor logic of each ofthe two or more processors. The apparatus further comprises a lock steplogic block including a processor-specific resource referenced by thecode sequence, the processor-specific recourse comprising a uniqueprocessor core identification, and a multiplexer coupled to theprocessor-specific resource, wherein the multiplexer is controlled toread data based on the identification. Coupled to the processors is alock step logic block operable to read and compare the output of each ofthe processors. The lock step logic determines if operation of theprocessors is in a lock step mode or in an independent processor mode.Such determination may be made by the lock step logic turning off, forexample.

The corresponding method for communicating between two or more lock stepprocessors comprises receiving a read instruction, determining aprocessor address associated with the read instruction, determining oneor more bits associated with the processor address, comparing the one ormore bits to the processor address to determine a location from whichthe read instruction is executed, and reading data from the determinedlocation.

DESCRIPTION OF THE DRAWINGS

The detailed description will refer to the following figures, in whichlike numbers refer to like elements, and in which:

FIG. 1 is a diagram of a prior art computer system employing lockstepped processors;

FIG. 2 is a diagram of a multi processor computer system employing lockstep processing in which two or more processors are able to directlycommunicate;

FIG. 3 illustrates the features of FIG. 2 in more detail;

FIG. 4 illustrates alternative means for inter-processor communication;and

FIG. 5 is a flow chart of an operation of the system of FIG. 3.

DETAILED DESCRIPTION

An apparatus, and a corresponding method, for communicating betweenprocessors in a multi-processor computer system are disclosed. Theapparatus and method may advantageously be applied to a computer systememploying lock step processors. Lock step processors, by definition, runidentical code streams, and produce identical outputs. FIG. 2 is adiagram of a computer system 100 that includes an apparatus that allowstwo or more processors to directly communicate without resorting toloads and stores. The computer system employs processor 110 (core 0) andprocessor 120 (core 1). The processors 110 and 120 may operate in lockstep. Coupled to the processors 110 and 120 is a lock step logic block130 and a cross-bar bus or arbitration unit 140 to controlcommunications with the rest of the system 100. Associated with theprocessor 110 is a multiplexer 112, a circular bus 116 and a read onlymachine specific register (MSR) 118. The bus 116 interfaces with thelock step logic block 130 through interface 114. Associated with theprocessor 120 is a multiplexer 122, a circular bus 126, and a read onlyMSR 128. The MSRs 118 and 128 and the multiplexers 112 and 122 provide ameans for data transfer.

The bus 126 interfaces with the lock step logic block 130 throughinterface 124. The lock step logic block 130 performs various checks toensure the processors 110 and 120 operate in lock step, when lock stepoperation is the desired mode of operation. Also coupled to theprocessors 110 and 120 is cross bar switch 140, which maps arbitraryinput ports to any arbitrary output port.

The arrangement of components shown in FIG. 2 is by way of example only,and is not meant to imply that a specific component must be located asshown. For example, the MSRs 118 and 128 are shown as part of theprocessors 110 and 120, respectively. However, the MSRs 118 and 128 maybe located outside the cores 0 and 1. Furthermore, the processors 110and 120, the multiplexers 112 and 122, and the lock step logic 130 mayall be located on a single silicon chip.

The processors 110 and 120 may, on occasion, need to communicatedirectly. However, conventional communication means (e.g., using loadand store instructions) will cause a loss of lock step operation.

To allow processor-to-processor communication, and not cause loss oflock step, communication means may be added to the system 100 shown inFIG. 2. Such communication means will incorporate the circular bases 116and 126. Use of the communication means establishes a logical connectionbetween the processors 110 and 120 such that one processor 110 is ableto read data for the other processor 120 using a simple code sequence.The use of an embodiment of the communication means is shown in FIG. 3.

In FIG. 3, the interface 114 for core 0 writes data to an MSR (600) 134and receives data from multiplexer 132. Similarly, the interface 124 forcore 1 writes data to MSR (600) 138 and reads data from the multiplexer136. In an embodiment, a least significant bit (lsb) of the MSR addressis used to determine which processor (110 and 120) the read data comesfrom. A “0” returns the “own” processor's data, while a “1” returns theother processor's data. This allows communications between theprocessors 110 and 120 with a code sequence such as:

-   -   write data to MSR [0X600]    -   read data from MSR [0X601].

More specifically, should the multiplexer 132 and the multiplexer 136receive the “0,” then the processor 110 will receive (read) data fromthe MSR (600) 134, and the processor 120 will read data from the MSR(600) 138. If the multiplexers 132 and 138 receive the “1,” then theprocessor 110 will read data from the MSR 138 and the processor 120 willread data from the MSR 134. Thus, by controlling the multiplexers 132and 136, communications between the processors 110 and 120 is possiblewithout loss of lock step.

Because only one bit is used, the technique can be used on an arbitrarynumber of register pairs, and can be extended to more than twoprocessors, by using a greater number of low-order bits in the addressof the MSR. Thus, in operation, both the processor 110 and the processor120 can write values out, and both processors 110 and 120 can read whatthe other processor wrote.

In the embodiment shown in FIG. 3, all components are located within thelock step logic 130. However, one or more of the components may belocated outside the lock step logic 130, while remaining coupled througheither the circular bus 116 or the circular bus 126. As an alternativeto the configuration shown in FIG. 3, other components may be used aspart of the communication means between the processors 110 and 120. Forexample, in place of the MSRs 134 and 138, the lock step logic 130 mayuse programmable resources, or may use non-programmable hard-wiredresources.

FIG. 3 shows a configuration to allow read communications between lockstepped processors. FIG. 4 shows a configuration to allow writecommunications between lock stepped processors without causing a loss oflock step. The configuration shown in FIG. 4 is identical to that ofFIG. 3, except that multiplexers 142 and 146 on the write side replacethe functions of the multiplexers 132 and 136 of FIG. 3 on the readside.

FIG. 5 is a flowchart illustrating an inter-processor communicationoperation 200 of the communication means of FIG. 3. In FIG. 5, theoperation 200 begins in block 205. In block 210, a read operation isspecified, with an address of the processor specified as a part of readoperation. The specific processor may be identified by comparing one ormore bits of the address to the addresses of the processors 110 and 120,block 215. For example, the least significant bit (lsb) may be used.Assuming the lsb of the address is “0,” the processor 110 reads the datafrom MSR(600) 134, and the processor 120 reads the data from theMSR(600) 138, block 220. In block 225, the operation 200 ends.

The terms and descriptions used herein are set forth by way ofillustration only and are not meant as limitations. Those skilled in theart will recognize that many variations are possible within the spiritand scope of the invention as defined in the following claims, and thereequivalents, in which all terms are to be understood in their broadestpossible sense unless otherwise indicated.

1. An apparatus for allowing direct read and write communication betweenlock step, processors, comprising: two or more processors operating in alock step mode, wherein each of the two or more processors comprise aprocessor logic to execute a code sequence, wherein an identical codesequence is executed by the processor logic of each of the two or moreprocessors; and a lock step logic block operable to read and compare theoutput of each of the two or more processors, the lock step logic blockcomprising: a processor-specific resource referenced by the codesequence, and a multiplexer coupled to the processor-specific resource,wherein the multiplexer is controlled to read data based on anidentification of the processor specific resource, the identification ofthe processor specific resource determining which processor supplies theread data, wherein the two or more processors operating in the lock stepmode can perform inter-processor read and write communications withoutresorting to load and store instructions that cause a loss of lock step.2. The apparatus of claim 1, wherein the processor-specific resource isa read-only machine specific register (MSR).
 3. The apparatus of claim1, wherein the processor-specific resource comprises a programmableregister.
 4. The apparatus of claim 1, wherein the processor-specificresource is hard-wired to a processor-unique value.
 5. The apparatus ofclaim 1, wherein the multiplexer is on a read side of a processor.
 6. Amethod for allowing direct read and write communication betweenprocessors operating in lock step, comprising: receiving a readinstruction; determining a processor address associated with the readinstruction; determining one or more bits associated with the processoraddress, the one or more bits specifying which processor supplies readdata; comparing the determined one or more bits to the processor addressto determine a location from which the read instruction is executed,wherein the determined location is a processor-specific resource; andallowing one processor to read data from the determined location foranother processor operating in lock step without resorting to load andstore instructions that cause a loss of lock step.
 7. The method ofclaim 6, wherein the processor-specific resource is a read-only machinespecific register (MSR).
 8. The method of claim 7, further comprisingexecuting a code sequence causes the processor address to be read fromthe MSR.
 9. The method of claim 6, wherein the processor-specificresource comprises a programmable register.
 10. The method of claim 6,wherein the processor-specific resource is hard-wired to aprocessor-unique value.
 11. The method of claim 6, wherein the one ormore bits comprise a least significant bit.
 12. An apparatus forallowing direct read and write communication between lock stepprocessors, comprising: a first communication bus for a first processor;a lock step logic coupled to the first communication bus; a secondcommunication bus for a second processor, the second communication buscoupled to the lock step logic, wherein the first and the secondprocessors operate in lock step; and means for communicating between thefirst and the second processors using the first and the secondcommunications buses, the communicating means allows the first and thesecond processors operating in lock step to perform inter-processor readand write communications without resorting to load and storeinstructions that cause a loss of lock step, wherein the communicatingmeans comprises: a first logic element for the first processor; a firstprocessor specific resource for the first processor; a second logicelement for the second processor; and a second processor specificresource for the second processor, wherein the first and the secondlogic elements are multiplexers.
 13. The apparatus of claim 12, whereinthe communicating means are incorporated into the lock step logic. 14.The apparatus of claim 12, wherein the first and the second processorspecific resources are machine specific registers.
 15. The apparatus ofclaim 12, wherein the first and the second logic elements compare one ormore bits associated with a processor address, and wherein thecomparison determines which of the processor specific resources is usedto supply read data to the first and the second processors.
 16. Theapparatus of claim 15, wherein the one or more bits comprise a leastsignificant bit of the processor address.
 17. The apparatus of claim 12,wherein the first and the processor specific resources are hard wiredresources.
 18. The apparatus of claim 12, wherein the communicatingmeans comprises means for communicating read instructions.
 19. Theapparatus of claim 12, wherein the communication means comprises meansfor communicating write instructions.